In colorimetric gas dosimeters a color-changing reagent is exposed to ambient air, usually through a diffusion path to minimize convection current errors, and the amount of color development is a measure of the dosage exposure to the detected gas. Dosage exposure is the integral over a time period of the concentration x time product. The "8-hour time weighted average" exposures to toxic gases, proscribed by OSHA as permissible exposure limits, is the dosage exposure over an 8-hour period divided by 8.
Two types of colorimetric gas dosimeters have heretofore been used. The length-of-stain type has an elongate bed of indicating chemical, usually with an active component on an inert granular carrier, disposed in a glass tube. Gas to be detected diffuses from an open end of the tube to and through the bed; a color change develops lengthwise of the bed, the length-of-stain being a measure of the dosage exposure. Such tubes have been widely used for measuring CO dosage. However, for many gases, and as exposure criteria are tightened, conventional length of stain tubes are not sufficiently sensitive to measure small doses. The length-of-stain developed for a given dosage is proportional to the amount of active component, but when the amount of active chemical is decreased to increase sensitivity, the intensity of the developed color also diminishes making it difficult to see the stain.
The color comparison type of dosimeter conventionally exposes an entire major surface of a reagent impregnated paper or similar substrate to the atmosphere. The exposure causes a spectrum of color change or variations depending on the dosage exposure. The exposed paper is compared to one or more color standards in order to read out the dosage exposure. Color comparison methods suffer inaccuracies because of wide individual subjective differences in comparing colors.